1. Field of the Invention
The present invention relates to a surface wave filter and, more particularly, to a low-loss filter having interdigital transducers and coupling transducers disposed on a substrate at selected distances from one another in two tracks.
2. Description of the Related Art
The surface wave filter which is disclosed in U.S. Pat. No. 4,468,642 is a bandpass filter, the transfer characteristic of which has low losses in the pass band and which has a high stopband attenuation and a steep edge slope. This known filter has two tracks with three or five or even more odd numbered interdigital transducers in each track, a track being the direction of wave propagation within the arrangement of transducers of each track. The two interdigital transducers at the end of each track have associated reflector arrangements which are provided for reflecting back the surface waves that have been sent out in the direction away from the transducer arrangements of the track from the end transducer, in such a manner that these end transducers act like unidirectional transducers. A coupling transducer is provided between the two end transducers of a track, which in the first track are, for example, input transducers. This also applies to the second track in which the transducers that are arranged on both sides of the relevant coupling transducer of the second track are the output transducers of the entire filter. Coupling between both tracks is effected by electrically connecting the one bus bar of a coupling transducer of the first track and the one bus bar of a coupling transducer of the second track. To obtain minimum loss, that is to say to obtain minimum matching attenuation, it is necessary that the two coupling transducers of the two tracks which are electrically connected to one another are very accurately matched to one another with respect to impedance. This requires identical coupling transducers. In practice, however, mismatching occurs to a certain extend which is based, among other things, on the effect described below.
Reference is made to the U.S. Pat. No. 4,468,642 for further details and for the operation of such a known bandpass filter in surface wave technology, as outlines above. FIGS. 1, 4 and 5 of the reference patent show embodiments and the further figures of the reference supply details on the transfer characteristics.
The drawing FIGURE attached to the present specification shows a very simple embodiment of such a known filter in which, the components contained within the dashed boundary form one cell of this filter. The cells can also be joined together in the case of the invention to form an embodiment according to FIG. 5 of the U.S. Pat. No. 4,468,642.
The cell 1 shown in the FIGURE has in the first track 2 two interdigital transducers 41 and 42, which are electrically connected in parallel and are to be operated, for example, as a filter input, and between the interdigital transducers 41 and 42 is a coupling transducer 43. Acoustic waves 4 and 5 that are excited and emitted by the transducers 41 and 42 are directed towards the coupling transducer 43 in a direction parallel to the track 2. In the coupling transducer 43, these acoustic waves 4 and 5 generate an electric signal (relative to ground) which occurs at circuit point DE (with respect to ground). The excitation by the acoustic waves 4 and 5 is such that, at the symmetric coupling transducer 43, these acoustic waves 4 and 5 must impinge on the coupling transducer 43 with identical phase. In the case of an anti-symmetric, or asymmetrical, coupling transducer 43, the impingement of the acoustic waves must be with opposite phases. The inter-digital transducers 41 and 42 are either symmetric or anti-symmetric transducers arranged at corresponding distances, relative to the coupling transducer 43. The arrangement of the transducers 51, 52 and 53 of track 3 is correspondingly the same as in track 2 with respect to phase. If the parallel-connected transducers 41 and 42 are the input of the filter, the parallel-connected transducers 51 and 52 are its output. The coupling transducer 53 is electrically excited starting from a circuit point D and sends out surface waves 6, 7 in the direction of transducers 51 and 52. If the coupling transducer 43 is symmetric, so is the coupling transducer 53. With an anti-symmetric coupling transducer 43, the coupling transducer 53 must also be anti-symmetric. Both coupling transducers 43 and 53 are of identical construction. If transducers 51, 52 and 53 are, for example, symmetric, surface waves 6 and 7 are allowed to impinge on the transducers 51 and 52 with the same phase, the distances d3, d4 being selected for this purpose. With an anti-symmetric transducer, distances matched to (2n-1) .lambda./2 must be provided, where .lambda. is the wavelength of the acoustic surface wave in the substrate. These measures are provided for matching. The reflector arrangements 40 have already been described above.
Such a filter in accordance with the FIGURE and in the manner of U.S. Pat. No. 4,468,642 shows disturbances in its transfer characteristic which are actually not expected and which impair the value of such a filter. These disturbances can be called mismatches but this does not describe their cause or even reveal a possibility for eliminating them.